GB2040899A

GB2040899A - Spent plasma reprocessing system

Info

Publication number: GB2040899A
Application number: GB7938419A
Authority: GB
Original assignee: European Atomic Energy Community Euratom
Current assignee: European Atomic Energy Community Euratom
Priority date: 1979-01-22
Filing date: 1979-11-06
Publication date: 1980-09-03
Also published as: US4304645A; EP0014077B1; DE3070607D1; JPS55127129A; JPS5838207B2; EP0014077A1; CA1147694A; GB2040899B

Description

1

GB 2 040 899 A

1

SPECIFICATION

Spent plasma reprocessing system

5 This invention relates to a process for the processing of spent plasma removed from a fusion reactor.

The process of this invention is applicableto a plasma formed by mixtures of deuterium and tritium contaminated by the helium produced by thefollow-10 ing fusion reaction:—

D + T + neutron

Other impurities are probably present in the plasma 15 stream such as CO, C02, N2, NO, N02, N(D,T)3,

C(D,T)4, C2(D,T)g having a very small concentration of about 2-3%. Even protium is expected to be present at a concentration of less than 1%.

All these impurities originate from many factors, 20 of which the major ones are the material degassing phenomena and the air infiltration even if in micro-quantities.

The prior art is replete with disclosures relating to the treatment of deuterium and tritium containing 25 the above-mentioned impurities which are expected to accumulate in a real fusion burn. In the known processing design for such a fuel cycle, it is assumed to make recourse to extremely costly and very advanced procedures such as selective impurities 30 cryogenic separation and hydrogen isotopes cryogenic distillation.

The overall dimension of these units and the other ones related to the process require a large facility for their containment.

35 Moreover for safety rules it is a compulsory requirement to have an emergency tritium clean-up system, in case of accident.

For this reaction, large investments and high running costs are demanded in order to maintain tritium 40 release into the atmosphere below the present levels which are becoming more and more strict.

The present invention deals with the discovery of a process which allows the complete cycle of the fuel from the removal step of exhaust plasma and its 45 impurities from the fusion reactor, the purification of the hydrogen isotopes from Helium and impurities, the oxidation of the hydrogen isotopes to their oxides, their distillation for removing the non-tritium oxide containing water and a mixture of deuterium -50 tritium oxides, their electrolysis to D2 and T2 and lastly final injection into the toroidal chamber after their molar composition have been correctly adjusted.

In accordance with the present invention, there is 55 provided a process for removing helium and other impurities from a mixture containing deuterium and tritium, which process comprises the steps of: separating from the mixture isotopes of hydrogen in any of their diatomic combined forms; oxidising the 60 separated isotopes to their corresponding oxides; separating tritium oxide and deuterium - tritium oxide from the oxides thus formed; and electrolysing the separated oxides to deuterium and tritium.

Preferably the oxides are separated by distillation 65 under reduced pressure and it is desirable to feed an excess of deuterium oxide to the distillation step.

A palladium membrane is preferably used to effect the isotope - separation step.

Waste products from the isotope - separation step may be oxidised and the oxidesed products adsorbed by a molecular sieve drier, before the unoxidised products are vented.

Preferably, waste products from the oxide -separating step are subjected to complementary purification by selective electrolysis in order to recover tritium in any of its diatomic combined forms; which recovered tritium is then recycled to the isotope - oxidising step.

In a preferred embodiment of the present invention the impure mixture of deuterium and tritium is a waste product of a fusion reactor, and the purified deuterium/tritium mixture is recycled to the reactor.

The invention also provides a deuterium/tritium mixture when purified in accordance with the process of the invention.

The process is applicableto all Tritium - Deuterium fusion reactors and for any choice that could be made about the operational procedure of the reactor, in other words if continuous or pulsed reactors are concerned. In the first case it is quite difficult to calculate the amount of tritium and deuterium to be processed per day, because it depends on different technological procedures (use of divertor or cold gas blanket), while for the second case the fuel will be about 2700 g/day of an equip-molecular quantity of D2 and T2 referring to a 2000 MW thermal power reactorto a burn/down ratio of 0.78 and a burn up of 10%.

That corresponds to a rate of 1.736g of fuel to be processed per minute. The fuel cycle can be more fully understood by reference to the accompanying drawing showing one embodiment of the present invention.

From the toroidal chamber -1 - of the fusion reactor, the exhaust plasma containing the impurities and He 'n a concentration between 0.1 to 30%, according to the burn up, is pumped out via flutter valve-2-and line-3-to cryosorption pump-4-.

Since the cryosorption pump is saturated, the gas condensed and absorbed is released by heating and transferred by a diaphragm or double bellows pump - 6 - by means of lines - 5 - and - 7 - to a sepa rator unit-8-

This unit allows the separation of the hydrogen isotopes from the impurities passing through a palladium membrane unit-8-almost completely. A second palladium membrane unit-10-is linked by a line-9-to unit-8-forthe purpose of improving the separation procedure.

The hydrogen isotopes are cycled by means of line -11 - to line-16-and then to burner-17-where the gas in presence of an excess of oxygen is transformed almost completely to the oxide form —18 -. What is not burnt up will be recycled by means of diaphragm pump-43-through line-19-. Avery effective oxidising catalytic reactor-45-is placed on the line-19-, in orderto transform quantitatively any residual hydrogen isotopes to their oxidised form. Different types of catalyst are used. Usually they are small pellets on which surface platinum.

70

75

80

85

90

95

100

105

110

115

120

125

130

2

GB 2 040 899 A

2

palladium orCuO have been deposited. Another candidate can be opcalite. The temperature can be maintained as low as 200°C, but in some cases, when traces of impurities as N(D,T)3 or C(D,T)4 are eventu-5 ally present, it must be brought to 400-500°C fortheir transformation to N2, C02, D20 and T20. In addition, a condenser—44 —is placed on the loop as to condense all D20, T20, and H20 (=£ 1%) formed in the burner and in the catalytic reactor- 20 -. 10 The helium and the impurities that contain tritium and deuterium even in the chemical form mentioned above, cannot be released to the atmosphere but must be treated in orderto recover all of the tritium. In this process, the gas is fed via line -12-to 15 another oxidising catalytic reactor -13-at 500°C and is transferred via lines-14-and-24-to the molecular sieve drier system-25-for the absorption of D20, T20, (H20==1%). The gas is then continuously recycled by transfer pump-46-until com-20 plete reaction is achieved. The residual gas - 26 - is then fed to a stack orto a waste disposal system.

In orderto minimize the water loading of these units, a condenser —15 - is placed down-stream of the catalytic reactor—13—for removing as much 25 water as possible.

The system-25-is made up of two molecular sieve driers in parallel. One of these is a stand-by and will be in operation whilst the other one (before reaching the breakthrough point) is being heated for 30 desorbing the water with the aid of a stream of dry nitrogen. The water-38 - is collected by a condenser— 36 — and, together with the water- 20,21 — from condensers-15-and-44-, is fed continuously by transfer pump - 22 - to distillation column -35 28 —by means of line-23—. The small input of water requires only a very small distillation column and consequently this allows a low inventory of tritium for the low hold-up of the column. The residual gases - 37 - from condenser- 36 - are vented. 40 In orderto reduce the tritium inventory further, one can make recourse to the method of adding an excess of liquid D20 to the column at reduced pressure, e.g. from 10 to 50 mm Hg pressure.

The principal aim of the distillation column is to 45 separate the major amount of the protium forms of water, H20, HDO from the other ones such as HTO, D20, DTO, T20. The small amount e.g. 1% of protium derivates, practically permits operation at total reflux for a long time and allows the occasional withdrawal 50 of the protium-rich minor portion of distillate. Another aim is to withdraw from the bottom mixtures of D20, T20, DTO. Experimental and complementary theoretical results suggest that the required separation can be achieved by using a col-55 umn of about 200 theoretical plates, 1.5 cm. diameter and 2 meter high, filled up with compressed wires.

The column works under vacuum in orderto increase the value of the separation coefficient. In 60 fact it ca n va ry fro m 1.0543 at 100°C to 1.19949 at 25°C as far as the mixture of H20 and T20 are concerned.

The (D,T)0 -27 - is transferred to electrolytic cell -33 — where it is decomposed to T2, D2 and 02. The 65 first two — 34 — are pumped by a diaphragm pump-

42 - to gas storage unit-39 - and then to fuel injection device-40-in orderto be fed to the toroidal chamber-1 - The 02 - 35 —which is used for the catalytic reactions, is recycled by diaphragm pump-41 —. The distillate, which is made up of H20, HDO and D20 together with small amounts of DTO of the order of 10-4% with respect to the total molar stream, is transferred by means of line-29-to electrolytic cell-30 —for complementary purification by the selective electrolysis of H2 and D2 from T2. This separation is feasible since the amount of tritium oxide in the cell is very small. This electrolysis is carried out by means of small cells using NaOH, H2S04 or another salt or ion exchange membrane as the electrolyte, and with anode electrodes made of nickel or iron/nickel. Such salts, in which the hydrogen is replaced by deuterium, are used for wetting membranes of, say, asbestos or similar porous materials which allow the separation of hydrogen from oxygen during electrolysis. Other materials and new sophisticated cells could also be used. For instance, the electrolytic cell 33 could operate in the vapour phase in orderto reduce the tritium inventory due to the hold-up of the same. I n this case, it is necessary to vaporize the feeding mixtures of D20-T20 downstream of the distillation column. An ion exchange membrane may be used in this method, a commercial version of which is called NAFION by Dupont. The residual liquid from the cell —30 —is returned to the column —28-by line—31 —.

H2, D2 with a trace of T2 are given off at the cathode of the cell-30 —and are vented-32—, whilst the 02 — 47-generated at the anode of the cell is recycled to the burner-17-

The equation governing the electrolytic separation of the hydrogen isotopes is easily derived. The heavy isotopes are discharged at the cathode more slowly than protium.

The effective separation depends on the values of a and which are the separation factors for deuterium and tritium with respect to protium, and vary sensibly according to the choice of electrolytes and electrodes. The known data are already high enough to attain a good separation especially in those cases when the tritium concentration is low. In the Figure only one cell is shown. A multistage operating cell could, however, be used in orderto meet the safety requirements for tritium release in the atmosphere or for its disposal.

In an alternative embodiment (not shown), a second distillation column can be used to assist the separation of the protium forms of water. In this case, the first column 28 separates a major proportion of the DTO and T20 from its distillate and, since the column need only have a comparatively small diameter for this separation duty, it requires a smal-lertritium inventory than for a single column. The final separation of H20 and HDO from the tritium forms of water is effected by feeding the distillate of the first column 28 into the middle of the second column, which, because of the reduced quantity of tritium-containing forms of water being handled therein, can be larger than the first column 28. The distillate from the second column is fed to the electrolytic cell 30, whilst the bottoms residue is either

70

75

80

85

90

95

100

105

110

115

120

125

130

3

GB 2 040 899 A

3

returned to the middle column 28 or is sent directto electrolytic cell 33 in accordance with the requirements of the process.

Claims

5 1. A process for removing helium and other impurities from a mixture containing deuterium and tritium, which process comprises the steps of: separating from the mixture isotopes of hydrogen in any of their diatomic combined forms; oxidising the

10 separated isotopes to their corresponding oxides; separating tritium oxide and deuterium-tritium oxide from the oxides thus formed; and electrolysing the separated oxides to deuterium and tritium.

2. A process as claimed in claim 1 wherein the

15 oxides are separated by distillation under reduced pressure and an excess of deuterium oxide is fed to the distillation step.

3. A process as claimed in claim 2 wherein a stream of liquid D20 is introduced to the distillation

20 step at from 10 to 50 mm Hg pressure so as favourably to affect the separation of protium from tritium and sensibly to reduce the tritium inventory.

4. A process as claimed in any one of the preceding claims wherein a palladium membrane is used to

25 effect the osotope-separation step.

5. A process as claimed in any one of the preceding claims wherein the waste products from the isotope-separation step are oxidised, and the oxidised products are adsorbed by a molecular sieve

30 drier before the unoxidised products are vented.

6. A process as claimed in any one of the preceding claims wherein the waste products from the oxide-separating step are subjected to complementary purification by selective electrolysis in orderto

35 recovertritium in any of its diatomic combined forms, which recovered tritium is then recycled to the isotope-oxidising step.

7. A process as claimed in any one of the preceding claims wherein most of the processing steps

40 handle the tritium in the form of its liquid oxides.

8. A process as claimed in any one of the preceding claims wherein the impure mixture of deuterium and tritium is a waste product of a fusion reactor, and wherein the purified deuterium/tritium mixture

45 is recycled to the reactor.

9. A process as claimed in claim 1 substantially as hereinbefore described with reference to the accompanying drawing.

10. A deuterium/tritium mixture when purified in

50 accordance with a process as claimed in any one of the preceding claims.

Printed for Her Majesty's Stationery Office by The Tweeddale Press Ltd., Berwick-upon-Tweed, 1980.

Published at the Patent Office, 25 Southampton Buildings, London, WC2A1 AY, from which copies may be obtained.